Similar to the record route option of the ping command, the traceroute command is used to
determine the path that the packet is taking through the network. However, traceroute uses a
different approach than the ping command. Specifically, the traceroute command starts by
sending out a packet with a time to live (TTL) of 1. The TTL of this packet will expire at the
first router, and therefore this device will send back a TTL expiration message. The address
from which the TTL expiration comes is then recorded, and a second packet is sent out with a
TTL of 2. The second-hop router then replies back with a TTL expiration message. This process
continues until the destination is reached.
The traceroute command operates in the Unix and Windows environment in the same manner
as the trace command in the Cisco router.
Though the functionality is the same, it is worthy of note that in the Cisco and
the Unix versions of traceroute, a UDP packet on port 33434 is used for the tracing,
whereas Windows stations use an ICMP echo instead.
In Windows, the syntax for the traceroute command is tracert. The options for the command
are shown in the following output, which is followed by a sample trace:
C:\>tracert /?
Usage: tracert [-d] [-h maximum_hops] [-j host-list] [-w timeout] target_name
Options:
-d Do not resolve addresses to hostnames.
-h maximum_hops Maximum number of hops to search for target.
-j host-list Loose source route along host-list.
-w timeout Wait timeout milliseconds for each reply.
C:\>tracert 10.5.5.5
Tracing route to 10.5.5.5 over a maximum of 30 hops
1 7 ms 1 ms 1 ms 10.21.2.1
2 84 ms 83 ms 84 ms 10.45.45.3
3 88 ms 85 ms 83 ms 10.10.10.67
4 87 ms 86 ms 88 ms 10.5.5.5
Trace complete.
When looking at the preceding trace, there are a couple things to note. First, Windows by
default sends out three traces for each TTL value. The times listed to the left of the IP address
are the times for each of the TTL expiration messages from these packets to return.
Also, when comparing the output from a recorded ping to the output of the traceroute command,
be aware of a couple of noteworthy differences. Ping records the exiting interface on the
router, whereas traceroute in general records the interface on which you enter. Another difference
is that when using a traceroute, you only get the path taken to the end device; you do not
see the return path.
As is the case with many of the commands discussed here, there are some subtle differences
between Unix and Windows in terms of both syntax and output. Here are the Unix command
options and a sample output:
unix1% traceroute
Usage: traceroute [-dFInvx] [-f first_ttl] [-g gateway | -r] [-i iface]
[-m max_ttl] [-p port] [-q nqueries] [-s src_addr] [-t tos]
[-w waittime] host [packetlen]
unix1% traceroute 10.5.5.5
traceroute to 10.5.5.5 (10.5.5.5), 30 hops max, 40 byte packets
1 10.21.2.1 (10.21.2.1) 1.046 ms 1.878 ms 1.880 ms
2 10.45.45.3 (10.45.45.3) 82.487 ms 84.850 ms 83.378 ms
3 10.10.10.67 (10.10.10.67) 84.196 ms 86.057 ms 84.105 ms
4 10.5.5.5 (10.5.5.5) 89.133 ms 88.664 ms 88.597 ms
unix1% 1081